1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a shadow mask frame assembly which is installed near a fluorescent screen on the inside of a panel of a color cathode-ray tube (CRT) and performs a color selection function, and a CRT having the same.
2. Description of the Related Art
FIG. 1 shows an example of a typical color CRT used for computer monitors and televisions. Referring to FIG. 1, the color CRT includes a panel 10 which has a fluorescent screen 11 on an inside surface of the CRT, a funnel 20 which is sealed to the panel 10 and has a neck portion 21 in which an electron gun 22 is installed, a deflection yoke 23 which is installed at a cone portion of the funnel 20, and a shadow mask frame assembly 30 including a shadow mask 31 which is separated from the fluorescent screen 11 by a predetermined distance. The shadow mask assembly 30 has a color selection function with respect to electron beams emitted from the electron gun 22 and a frame 39 which supports the shadow mask 31.
In a color CRT having the structure described above, electron beams emitted from the electron gun 22 are deflected by the deflection yoke 23, pass through electron beam passage holes formed in the shadow mask 31, and strike red, green, and blue fluorescent materials of the fluorescent screen 11 to excite the fluorescent materials, thereby forming an image.
In the above-described operation, if the electron beam passage holes in the shadow mask 31 move from a specific position relative to the screen 11, electron beams cannot precisely strike the corresponding fluorescent materials. Accordingly, it is necessary to prevent the structural change or misalignment of the electron beam passage holes while manufacturing the shadow mask frame assembly 30, to maintain optimal color selection for a distinctive image.
As shown in FIG. 2, the shadow mask 31 is formed by press-molding a thin film having a plurality of holes in a predetermined area. The shadow mask 31 includes an apertured portion 32 with a plurality of electron beam passage holes, an imperforate portion 33 extending from the edge of the apertured portion 32, and a skirt portion 34 which is vertically bent down from the imperforate portion 33. The shadow mask 30 is made of killed steel or invar and thus is not easily molded. Particularly, the skirt portion 34, which is bent at a right angle to the imperforate portion 33, flexes back toward a flat position due to a spring-back phenomenon. Thus, the inherent elasticity of the material inclines the skirt portion 34 away from the center of the shadow mask 31.
The shadow mask 31 is fixed to the frame 39 by pressing the skirt portion 34 inward and inserting the skirt portion 34 behind the frame 39. The shadow mask then is welded to the frame 39. When the skirt portion 33 of the shadow mask 31 is pressed inward in the direction D shown by the arrow in FIG. 2, a bending stress causes changes in the imperforate portion 33 and the apertured portion. Since the apertured portion 32 is weakened due to the electron beam holes, the apertured portion 32 may flex and experience a curvature, as shown in path A of FIG. 2. While the curvature may not be visibly noticeable, it can displace electron beam passage holes at the periphery of the apertured portion 32 from originally set positions. Consequently, in a color CRT using this shadow mask 31, electron beams passing through the electron beam passage holes may not strike the fluorescent screen in the correct locations due to misalignment, thereby degrading the white balance characteristic of an image.
The structure of a color shadow mask for overcoming the above-described problem is disclosed in U.S. Pat. Nos. 6,111,346 and 6,274,974 B1. As shown in FIGS. 3A through 4B, a shadow mask 40 includes a curved apertured portion 41 having a plurality of electron-transmissive apertures, an imperforate portion 42 surrounding the curved aperture portion 41, and a skirt portion 43 which is bent back from a periphery of the imperforate portion 42 at long and short sides. The skirt portion 43 is spot-welded to a frame (not shown) so that the shadow mask 40 can be supported by the frame. The skirt portion 43 includes a plurality of slits 44 and embossments 45 which extend in a direction of a height of the skirt portion 43.
This conventional shadow mask 40 has a plurality of embossments 45 and slits 44 in the skirt portion 43 so that the spring-back phenomenon of the skirt portion 43 can be prevented during manufacturing of the shadow mask 40. However, as shown in FIG. 5, since the contour of the skirt portion 43 is complicated due to the plurality of slits 44, it is not easy to insert the skirt portion 43 into a frame 47. In addition, the slits 44 may be distorted by damage from a carrier during manufacturing. Moreover, formation of the slits 44 produces many cut-away pieces, which can contaminate products.
Another shadow mask is disclosed in Japanese Patent Publication No. 57-105946. The shadow mask includes a half etched portion in which a skirt portion is formed using etched and a half non-etched portion surrounding the half etched portion (not shown).
A structure in which embossments are formed in a skirt portion of a shadow mask in the height direction of the skirt portion is disclosed in Japanese Patent Publication No. 1-197,942.
A structure in which holes such as slits are formed in a skirt portion of a shadow mask is disclosed in Japanese Patent Publication No. 8-298,078.
Methods of manufacturing a mask are disclosed in U.S. Pat. Nos. 4,094,678 and 4,210,843.